18.6: Evolutionary origins of clonal (permanent) multicellularity

An interesting aspect of the unicellular-multicellular-unicellular behaviors of social slime molds is that evolutionary selection acts on both uni- and multi-cellular stages. A major evolutionary transition, leading to the appearance of permanently multicellular plants, animals, and fungi is estimated to have occurred some time in the Cryogenian period (834–780 Ma).^[598] Exactly how this transition occurred, on how many occasions, and exactly why remains unclear - presumably it involved selection for organisms that could exploit a new range of ecological niches.

Aggregative multicellularity involves an extension of quorum sensing and social cooperation between genetically distinct but related individuals. We can speculate on the drivers of true multicellularity, in which all cells of the organism are clonally related and one generation is related to the next through meiosis, gamete formation, and fusion.^[599] Two plausible adaptive (evolutionary) drivers of permanent multicellularity spring to mind: as a way to avoid or discourage predators by getting bigger and as a way to produce varied structures needed to more efficiently exploit ecological niches and life styles, opportunities not available or as efficiently exploited by unicellular organisms.

An example of the first type of driver is offered by the studies of Boraas et al; they cultured the unicellular green alga Chlorella vulgaris, together with a unicellular predator, the phagotrophic flagellated protist Ochromonas vallescia.^[600] After less than 100 generations they observed the appearance of multicellular, and presumably inedible (or at least less easily edible), forms of Chlorella. Once selected, this trait appeared stable, such that “colonies retained the eight-celled form indefinitely in continuous culture”. At the time of this writing the genetic basis for this multicellularity remains to be determined.

Another relevant evolution of multicellularity experiment are the studies by Ratcliff et al. They selected yeast that failed to separate after mitosis – the resulting clumps of cells fell to the bottom of culture tubes, forming multicellular aggregates known as "snowflakes". Within each snowflake cluster, the cells were linked mother to daughter in chains. A snowflake would "divide" into two when one of the cells in a chain died by programmed cell death (apoptosis).^[601] This type of division is quite distinct from the process of sexual reproduction. The evolution of true multicellularity involves the appearance of specialized cells involved in reproduction, the germ line-somatic divide. ^[602]

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